The present invention relates to a semiconductor element and a liquid crystal display, and in particular, it relates to a semiconductor element, suitable for use as a thin film semiconductor element, and an active matrix addressing type liquid crystal display using the same.
As display devices for displaying image information and characters for use in the field of office automation equipment and the like, active matrix addressing liquid crystal displays using thin film transistors (hereinafter referred to as TFT) have been known. For this type of conventional liquid crystal display, further improvements in resolution and high quality of display, as well as cost reduction in manufacture, are necessary. In order to solve these problems, it is most important to improve the performance of the thin film transistor, which is a key device to the active matrix addressing liquid crystal display. In this regard, in the Conference Record of International Display Research Conference 1985, page 9, a concept of forming advanced high performance TFTs on a glass substrate was proposed, in which peripheral drive circuits for driving a TFT active matrix are fabricated using TFTs and are integrated with its display unit on the same substrate so as to reduce the cost of manufacture. With provision of advanced high performance peripheral drive circuits integrated on the glass substrate, since other circuits to be mounted externally and their mounting process can be simplified, a substantial reduction in the packaging cost can be expected. However, in order to provide these high performance circuits, development of a more improved, sophisticated high performance TFT is required. In particular, TFTs, which are formed on polycrystalline silicon (hereinafter referred to as poly-Si) and appear to be the most promising TFTs suitable for use in display equipment having integrated peripheral drive circuits, have further technical problems yet to be solved, which problems include the need for improvement of the carrier mobility and a lowering of the threshold voltage Vth.
A method of operation at a lower voltage by lowering the threshold voltage Vth of the transistors formed on an insulator has been proposed, as described in the Technical Digest of International Electron Device Meeting 1994, pages 809-812, in which a so-called dynamic threshold voltage MOSFET (hereinafter referred to as DTMOS) which has a fourth contact, separate from the source and drain, on a semiconductor film acting as a bipolar active layer, and is in contact with the gate electrode, is proposed as a submicron silicon-on-insulator (SOI) device.
In the technique described in the above-mentioned publication, the threshold voltage of the transistors is substantially reduced further to lower the operating voltage of the circuits thereof, thereby lowering the power supply voltage, and thus minimizing the power consumption in the drive circuits. This feature of decreased power consumption achieved by this drive circuit is desirable for use as a drive circuit and device for a liquid crystal display as well. It should be noted, however, that the above-mentioned publication is directed to the provision of logic devices or memory devices that can be operated at a low voltage and a high speed, and so they cannot be applied, as they are, to the drive devices for driving a liquid crystal display.
Namely, the semiconductor device, as described in the aforementioned publication at pages 809-812, is designed to operate effectively with a gate voltage at 0.6 V or less, and so this semiconductor device cannot be applied as it is to a liquid crystal display in which a lower limit of the gate drive voltage is determined by the liquid crystal drive voltage. More particularly, at least .+-.3 V in peak amplitude is required for driving normal liquid crystal materials, thereby a higher voltage than this minimum peak voltage is required as the gate voltage for driving pixel drive transistors in a liquid crystal device. Thus, the semiconductor device described above, which operates at the gate voltage less than 1 V, cannot be applied to a liquid crystal display.